X-ray inspection systems are presently limited in their ability to detect and distinguish contraband, drugs, weapons, explosives, and other items of interest concealed in cargo from benign materials. There is further an interest in inspecting cargo for manifest verification purposes to ensure appropriate customs duty is paid.
The intensity of the transmitted X-rays is related to the areal density (i.e. density×thickness) and the atomic number (Z) of the materials they traverse. Radiographs produced by conventional X-ray systems are often difficult to interpret, because objects are superimposed and no Z information is provided. Therefore, a trained operator must study and interpret each image to render an opinion on whether or not a target of interest, or a threat, is present. When a large number of such radiographs are to be interpreted, such as at high-traffic transit points and ports, these inherent difficulties, combined with operator/screener fatigue and distraction, can compromise detection performance. There is a need for automatic detection and/or screener-assist tools for detection of threats and other targets, in order to improve the efficiency and accuracy of operators, and to reduce the number of operators needed for the detection.
Methods known to those skilled in the art for obtaining useful Z-information include the use of dual-energy X-ray sources, and dual-species technologies (X-ray inspection combined with neutron inspection). However, these methods do not readily allow accurate determination of the actual Z of the cargo contents, but rather yields an average Z that represents a mix of the various materials in the X-ray beam path. Thus, these methods are not efficient.
Therefore, X-ray inspection systems currently available in the art provide limited accuracy for detection of items of interest. Further, these systems do not effectively detect high atomic-number (“high-Z”) materials. Detecting such materials, particularly smuggled special nuclear materials (SNM) that could potentially be used to make a weapon, or materials used to shield their radioactive emissions, is a very complex task. One of the materials of greatest concern, highly enriched uranium (HEU), has a relatively low level of radioactivity. Plutonium, another nuclear weapons grade material, has a higher specific activity and higher-energy emissions. However, it can be shielded by employing a combination of high-Z materials for shielding gamma rays and low-atomic number (“low-Z”) neutron absorbers for shielding neutrons produced by spontaneous fission. Thus, it is very difficult to detect shielded or concealed materials.
It is therefore desirable to have improved methods and systems for effectively detecting high-Z materials, particularly accounting for the possibility that such materials may be shielded by a combination of high-Z materials for shielding gamma rays and low-Z neutron absorbers for shielding neutrons.